1. Field of the Invention
The present invention relates to a lanthanum phosphate phosphor for vacuum ultraviolet radiation, which exhibits light emission of high luminance when irradiated with vacuum ultraviolet radiation having a wavelength of at most 200 nm and which is less susceptible to deterioration of emission luminance by an increase of the temperature (temperature quenching), and a rare gas discharge fluorescent lamp (rare gas lamp) which is less susceptible to deterioration of the luminance with time, wherein such a phosphor is used as a fluorescent layer.
2. Discussion of Background
A rare gas lamp is a fluorescent lamp having a rare gas such as Xe, Ar or Xexe2x80x94Ar sealed in instead of mercury, so that an inside fluorescent layer is excited to emit light by vacuum ultraviolet radiation having a wavelength of at most 200 nm, which is radiated by discharge of such a rare gas. With a fluorescent lamp having mercury sealed in, the saturation vapor pressure of mercury changes due to a change in the surrounding temperature and in the surface temperature of the tube, whereby the emission output from the fluorescent layer changes substantially. Whereas, with the rare gas lamp, the change in the gas density due to a difference in the temperature is little, whereby it can be lighted up instantaneously, and a constant emission output can be obtained. Further, mercury as a hazardous substance is not used, and no environmental pollution results from disposal of such a lamp. Accordingly, it is presently widely used as a light source for information related equipments, such as a light source for reading out of facsimile or a back light for a liquid crystal display.
The rare gas lamp is required not only to have high luminance but also to have little decrease of light flux due to an increase of the surrounding temperature, when it is used as a light source for reading out a manuscript of facsimile. Accordingly, the phosphor to be used as a fluorescent layer of a rare gas lamp is required not only to emit light at high efficiency under excitation with vacuum ultraviolet radiation having a wavelength of at most 200 nm, but also to have little temperature quenching i.e. little decrease in the emission luminance due to an increase of the temperature.
As phosphors for rare gas lamps, a green-emitting phosphor such as Zn2SiO4:Mn, LaPO4:Ce,Tb or Y2SiO5:Tb, a blue-emitting phosphor such as BaMgAl10O17:Eu or (Ba,Sr)MgAl10O17:Eu,Mn, and a red-emitting phosphor Y2O3:Eu or (Y,Gd)BO3:Eu, are, for example, presently used. Among these phosphors, a lanthanum phosphate phosphor (LaPO4:Ce,Tb) is particularly excellent in that emission luminance under excitation with vacuum ultraviolet radiation is high. However, when this phosphor is used as a fluorescent layer for a lamp, temperature quenching becomes distinct when subjected to heat treatment such as baking treatment in the process for forming the fluorescent layer, and it has a drawback that when the lamp is continuously turned on, as the temperature of the lamp increases, the light flux of the lamp decreases, and the emission luminance decreases. Therefore, an improvement in this respect has been desired.
The present invention has been made under the above circumstances, and it is an object of the present invention to provide a lanthanum phosphate phosphor for vacuum ultraviolet radiation, whereby there is no temperature quenching and the emission luminance does not substantially decrease even when the temperature rises, and a rare gas lamp whereby the temperature dependency of light flux is little.
In order to accomplish the above object, the present inventor has studied the interrelation between the composition of the lanthanum phosphate phosphor which has been heretofore used and the temperature quenching of the phosphor, in detail. As a result, he has found it possible to remarkably reduce the temperature quenching by adjusting the concentration of the activator, particularly cerium (Ce) and to obtain a rare gas lamp, whereby the decrease of light flux due to a temperature rise of the lamp with time, is little, by using such a phosphor as a fluorescent layer. The present invention has been accomplished on the basis of this discovery.
Thus, the object of the present invention can be accomplished by adopting the following construction.
(1) A lanthanum phosphate phosphor for vacuum ultraviolet radiation, which is represented by a composition formula of (La1-x-yCexTby)PO4, wherein x and y are numbers which satisfy 0xe2x89xa6xxe2x89xa60.2 and 0.05xe2x89xa6yxe2x89xa60.3, respectively, and which emits light when irradiated with vacuum ultraviolet radiation having a wavelength of at most 200 nm.
(2) The lanthanum phosphate phosphor for vacuum ultraviolet radiation according to the above (1), wherein x is a number which satisfies 0xe2x89xa6xxe2x89xa60.15.
(3) The lanthanum phosphate phosphor for vacuum ultraviolet radiation according to the above (1) or (2), wherein the wavelength of the vacuum ultraviolet radiation is 172 nm.
(4) A lanthanum phosphate phosphor for vacuum ultraviolet radiation, which comprises lanthanum phosphate (LaPO4), as a matrix, and terbium (Tb), or cerium (Ce) and terbium (Tb), as an activator, and of which the emission luminance at 150xc2x0 C. after baking treatment at 800xc2x0 C. for 20 minutes under excitation with vacuum ultraviolet radiation having a wavelength of at most 200 nm, is within a range of from 100 to 80% of the emission luminance of the phosphor at 25xc2x0 C. before the baking treatment.
(5) The lanthanum phosphate phosphor for vacuum ultraviolet radiation according to the above (4), which is represented by a composition formula of (La1-x-y CexTby)PO4, wherein x and y are numbers which satisfy 0xe2x89xa6xxe2x89xa60.2 and 0.05xe2x89xa6yxe2x89xa60.3, respectively.
(6) The lanthanum phosphate phosphor for vacuum ultraviolet radiation according to the above (4) or (5), wherein the wavelength of the vacuum ultraviolet radiation is 172 nm.
(7) The lanthanum phosphate phosphor for vacuum ultraviolet radiation according to any one of the above (1) to (6), wherein the average particle size of the above phosphor is from 0.5 to 3.0 xcexcm.
(8) A rare gas discharge fluorescent lamp comprising a tubular enclosure transparent to light, having a fluorescent layer formed on an inner wall thereof, and a rare gas sealed in the enclosure, so that the fluorescent layer is excited to emit light by vacuum ultraviolet radiation having a wavelength of at most 200 nm which is radiated by discharge of the rare gas, wherein the fluorescent layer is made of the lanthanum phosphate phosphor for vacuum ultraviolet radiation as defined in any one of the above (1) to (7).
(9) The rare gas discharge fluorescent lamp according to the above (8), wherein the rare gas comprises xenon (Xe).